Abstract

Gold, bismuth and copper mineralisation at Tennant Creek occursin transgressive magnetite- and hematite-rich lodes within the CarramanFormation of the Lower Proterozoic Warramunga Group.Rocks of the Warramunga Group are dominantly felsic greywackesand shales with features indicative of turbidity current deposition. Theseare interbedded with massive pyroclastic rocks, rhyolitic lavas, preconsolidationslump breccias and minor lenses of banded iron formation.The magnetite-hematite lodes (locally referred to as ironstones)have an ellipsoidal to pipe-like shape commonly flattened in the directionof the regional east-west cleavage. They are typically localised insmall anticlinal structures within the greywacke-shale turbidites adjacentto thin lenses of hematite-chlorite-calcite bearing banded iron formationsor hematite-rich shales. A smaller number of mineralised ironstones havereplaced the preconsolidation slump breccia horizons within the felsicsediment pile.Silicate, oxide and carbonate gangue minerals within the lodestructures are grouped into a series of compositionally distinct zoneswhich exhibit sharp contacts against one another and the enclosingcountry rocks , Massive magnetite (>80%) and Fe-Mg chlorite (<20%)commonly constitute the core of the mineralised lode and are surroundedby vari ous umbrella-shaped zones. These may be: talc-magnet.ite; dolomite;chloritised sediments, as at J·uno Mine, or quartz-hematite; hematitemagnetite;hematite-chlorite; chloritised sediments, as at Gecko Mine.The magnesium content of the chlorites (dominantly ripidolites) increasesfrom the base, to the top, of the lode structures. The chemical and mineralogical characteristics of these zonesindicate contemporaneous formation and growth, resulting from the flow ofhydrothermal solutions which reacted with the host rocks and sufferedcontinual, and systematic, changes in chemistry. A zone of intensechloritisation extends below each of the orebodies and constitutes whatis thought to be a channel of hydrothermal alteration.This investigation deals with the structure, mineralogicalconstitution, mineral zoning, textures, and origin of the lode rocks inthe three largest operating mines in the goldfield, namely the Juno, Geckoand Warrego deposits.At the Juno and Warrego mines, gold, bismuth and coppermineralisation has been shown to occur in three overlapping zones withinthe magnetite-rich lodes e Gold is concentrated at depth, and is overlainabove by an umbrella shaped zone rich in bismuth sulphosalts. Chalcopyriteis concentrated at the top of the lode structures enveloping thebismuth zone. At Gecko (anomaly 2), bismuth and copper show a similarvertical zonation but gold is completely lacking. Within the bismuth zoneat Juno, the sulphur/selenium and bismuth/ lead ratios of the bismuthsulphosalts increase from its inner edge (overlapping the gold zone) toits outer edge (overlapping the copper zone).The most common bismuth sulphosalt at Juno is junite, a newmineral, unique to Juno, which has been shown by microprobe analysis tohave the formula Bi8PbJCu2 (S,Se) 16 , containing 3.8 to 11.6 wt.% selenium.Junite is easily distinguished from other lead-bismuth sulphosalts by itscharacteristic x-ray powder pattern. The second most abundant bismuthsulphosalt has a composition close to Bi10Pbg(S,Se) 23 and may be equivalentto the mineral wittite , previously reported from Falun, Sweden byJohansson in 1924 . Other selenium bearing sulphosalts at Juno includeheyrovskyite and members of the aikinite-bismuthinite series.Colloform textures occur throughout all the lode structures in the goldfield, strongly indicating that replacement of the sediment hostrocks to form the ironstone bodies was achieved by the processes ofdispersive metasomatism. Replacement of this type involves the gradualhydrolysisMg 2+ 1' .n t h eof the host sediments and permits ready exchange of Fe 2+ andhydrothermal solutions with si4+, Al3+, Na+ and K+ in thehydrolysed sediment matrix. Most of the magnetite in the central lodezones replaced needle-shaped a - FeO(OH) and S - Fe203.H20 crystal formswhich probably developed from ageing of ferric hydroxide gels.Thermodynamic considerations of gangue mineral stabilities inhydrothermal solutions of the type which may have caused mineralisationat Tennant Creek, suggests that the solutions were initially acidic innature and capable of 1 each1•. ng the I base catl.' ons I Fe 2+ and Mg 2+ (p 1 us oremetals) from the Carraman sediments at depth. There is evidence of leachingof this type in the lowest parts of the Juno hydrothermal channel.The process of metasomatic lode formation was most probably initiated byinteraction of rising, chloride-rich, hydrothermal solutions with thecalcite bearing banded_ iron formation or with particularly porous horizonscontaining abundant pore fluids (e.g., preconsolidation slump breccias)leading to an increase in solution pH and f02 which caused the depositionof amorphous, hydrated, ferric oxides.Under the influence of increasing solution p~the gangue mineralsat Juno, were deposited in the order:- iron-rich chlorite at depth,followed by hematite, magnetite, talc and dolomite, to form a well zonedlode structur e . The progressive drop in f02 associated with the depositionof magnetite probably resulted in the increase in sulphur/selenium andsulphur/metal ratios of sulphides passing up the Juno lode structure, andmay have also lead to the zonal distribution of gold, bismuth and copper .Sulphur isotope studies at Juno lend support to this proposal.The16o;18o and12c;13c ratios in dolomites from the outerenvelope zone at The 32 S/34 S ratlio of syngenetic sulphides in the tuffaceous greywackes and vein sulphides in the hydrothermal channel,below the orebody, supports a proposal that the sediments of the LowerCarraman Formation provided the source for the sulphur. These sedimentsalso contain sufficient trace quantities of gold, bismuth and copper toconstitute a source for the ore components.Connate wate.rs (pore water and interlayer water) released fromthe argillaceous sediments in the vicinity of granitic and rhyoliticporphyry intrusions provides the most probable source for the hydrothermalsolutions. Such solutions moved upwards continually leaching iron,magnesium and ore elements from the sediments in their path and wereeventually channelled into low pressure anticlinal sites and depositedtheir metal load in favourable structural-lithological traps.

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